1. Field of the Invention
The present methods, devices, and systems relate generally to the field of radars, and more particularly to HF/VHF radars that scatter signals from ocean surface or from targets such as ships on the sea. Specifically, the present methods, devices, and systems invention relate to antenna systems useful for such radars. The present methods, devices, and systems facilitate reduction in antenna system size while providing the level of performance found in current larger antenna systems.
2. Description of the Related Art
HF radars have been used since the 1960s. When located at coastal areas and transmitting vertical polarization, HF radar systems may exploit the high conductivity of sea water to propagate their signals (e.g., in a surface-wave mode) well beyond the visible or microwave-radar horizon. Although HF surface-wave radar (HFSWR) was initially considered for detecting military targets beyond the horizon (e.g., ships, low-flying aircraft or missiles), HFSWR also found widespread acceptance and use in the mapping of sea surface currents and the monitoring of sea state (e.g., waveheights). The radar echo used in these sea mapping/monitoring applications comes from Bragg scatter by ocean surface waves that are about half the radar wavelength, traveling toward and away from the radar.
Conventional radars determine target bearing by forming and scanning narrow beams using radar antennas. One procedure for sea mapping/monitoring using HFSWR has been to use a transmit antenna system that floodlights a large bearing sector of the sea (e.g., 60°) with illumination. A separate receive phased-array then forms a narrow beam that is scanned across the illuminated sector using software algorithms after signal digitization. The beamwidth (i.e., angular resolution) depends on the length of the antenna aperture, being proportional in radians to the wavelength divided by the array length. Because the wavelength at HF may be almost 1000 times greater than for microwave radars, the length of an HF array may be hundreds of meters long. While such radars were built and operated in the 1960s, antenna size and related cost impeded widespread acceptance. Coastal locations are valuable land for other public and private use, and suitable locations for large antennas as coastal structures are difficult to obtain.
Compact HF radar systems may take the place of the above-described large phased arrays. CODAR systems have employed separate transmit and receive antenna subsystems, with the two units separated by up to a wavelength. In many cases, such structures were still considered to be too obtrusive, and therefore incompatible with public use in beach areas, or for deployment on oil platforms or building rooftops.
These compact antenna systems for sea mapping/monitoring coastal radars included separate transmit and receive antenna subsystems. The transmit unit was usually an omni-directional monopole, and the receive unit consisted of two crossed loops coaxially collocated on a vertical monopole. Such antenna systems were sufficiently compact that they were suitable for mounting on offshore oil platforms and on coastal building rooftops. Reductions in size may be achieved by replacing the large air loops employed by earlier technology with tiny crossed ferrite loopsticks housed in a weatherproof box on the post surrounding the monopole.
The loopstick antennas take advantage of the fact that an inefficient HF receive system will cause reduction of the desired target signal as well as a proportional reduction in the external noise. Therefore a signal to noise ratio (SNR) of the HF receive system may remain constant with decreased efficiency, to the point where the external noise is approaches the internal receiver noise, at which point SNR begins to suffer. Thus, the size and cost of the HF receiver antenna subsystem can be reduced (thereby decreasing its efficiency) to the point that the external noise approaches the internal receiver noise before any SNR penalty is experienced by the HF receiver antenna subsystem.
Coastal space available for radar antenna systems continues to shrink, and further reductions in size are desired. Coupling between transmit and receive antennas in a radar system reduce performance of the radar antenna system. Furthermore, external obstacles nearby such as power lines, buildings, fences, and trees all exacerbate mutual coupling problems.